I'm trying to build navigation system for a lawn mower. I've tried different methods of navigation (RTK GPS, DW1000 radio tags) but they all have drawbacks (like RTK GPS losing his "fix" under trees and near walls). Looks like april tags could help me in that but seems raspberry pi zero (which is mounted on the robot) does recognize those at very slow speed, like more than one second with 640x480 captures from camera. The robot doesn't move very fast but still needs to recognize tags 3-5 times per second for successful navigation by them. Do the openMV can provide such speed at 640x480 or better resolution?

We run at 160x120 at 10 FPS with the M7. 160x120 provides about 8 ft of distance for a tag that's 8"x8".

Lower your res on your Pi if you want faster speed. The algorithm only needs that for distance. If your field of view is constrained then use lenses (can't do this with a Pi) to focus on a narrower area in front of you for faster response.